Beamline Optics and Deceleration

The size, shape
and position of the beam spot at the sample position is measured by
installing a plastic scintillator (BC412) in the sample position (in
either spectrometer) and imaging the light emitted by the scintillator
with a CCD camera. Such
images are used for the final stage of beamline tuning to ensure good
focussing and position of the beamspot over a range of magnetic fields
and implantation energies.

CCD beam spot imaging
system.

In order to
precisely focus and steer the radioactive ion beam spot onto small
samples we have developed an inexpensive yet versatile CCD system to
image the light produced when the ion beam is stopped in a piece of
scintillator mounted in a sample holder. This system allows one to
nearly make real time adjustments to the electrostatic optical elements
which define the shape and position of the beam spot.

Figure 1: A typical
black and
whit beam spot image. The contrast and brightness have been modified
such that all features become apparent. The square shape outside the
beamspot is formed by a copper heat shield that has been placed at the
end of the cryostat.

The heart of this
system is a 16-bit monochrome CCD camera (model MX516 manufactured by Starlight Xpress, UK)
incorporating a low-noise CCD and Peltier cooler. The MX516's low dark
current allows for long exposures to be made. A 50mm f/1.4 lens
provides
a narrow 7 degree field of view on the 510 x 290 pixel CCD. Peak
sensitivity of the CCD ocurrs at 500-550 nm, close to the emission
wavelength of several types of plastic and inorganic (UHV compatible)
scintillation detectors.

The camera is
mounted outside the UHV chamber but views the sample face almost along
the beam axis through a UHV viewport and via a front surface mirror
mounted inside the UHV chamber which provides an approximately 90
degree
bend in the optical path.

For each 8Li
ion stopped, energy is deposited into the scintillator via three
channels. The initial beam energy first contributes up to 30 keV per
stopped ion. Secondly, when the ion decays the energetic beta
contributes an amount of energy that depends on its path length in
scintillator, averaging a hundred keV or so. Finally, two very
short-ranged alpha particles produced in the decay of 8Be
from an excited state each contribute 1.6 MeV.

Exposures of 30
seconds are sufficient to record the 8Li beam spot with an
ion flux of about 107 per second incident on 0.25mm thick
Bicron BC412 plastic scintillator. An example image of the beam spot is
shown above. In the picture the beam spot has a FWHM of (1.1 ±
0.2)mm. With longer exposures it is also possible to image the light
produced by much more intense beams of stable ions obtained from an
off-line ion source.

Using the source
code provided by Starlight
Xpress, we have created a small program that enables conversion
from the original fits image format to an ascii data file. This file is
then imported into PHYSICA to
allow for further qualitative and quantitative image analysis.

From the black and
white image we can see where the majority of the beam energy is being
deposited. However, without extensive contrast manipulation it is
difficult to understand more specific details such as the beam
intensity
peak shape. A surface plot of the entire image matrix or a cross
section plot of one of the rows/columns reveals that the peak normally
follows a Gaussian distribution. This is shown below in figures 2 and 3.

Figure 2: Surface plot
of the
same image as in Figure 1.

Figure 3: Cross-section
graph of
the same image as in Figure 1. The data was obtained by averaging over
all angles around the peak. The red line represents a fit to the sum of
two Gaussian distributions.

Implantation Depth and Deceleration

Typically the ion
beam has an energy of about 30.5 keV which is modified slightly by the
sodium cell bias. To control the ion
implantation
range, one can adjust the beam energy. In practice this is done by
raising the entire experiment (sample, cryostat, magnet, rf system, daq
computer ...) to a high positive voltage on the "platform". From the
point of view of the ions in the beam this presents a potential hill
that they must climb at the expense of their longitudinal velocity.

A radial section
of the end of the beamline in the high field spectrometer (in the bore
of the solenoid) is shown below. The contours indicate 10% of the range
of the potential difference between the red areas (shorted to the
platform) and the green area (shorted to ground). The potential map was
made by numerical solution of Laplace's equation in cylindrical
geometry
using the program RELAX3D.
The beam incides from the right.

Potential maps
such as this are used to predict how the beam is transported into the
sample, i.e. the terminal beamspot, and how this is modified by
upstream
electrostatic beamline optics. The high magnetic field of the
superconducting solenoid has a significant effect on this transport and
is included in such calculations.

Looking along the
axis (r=0), an example of the potential hill presented to the beam is
shown below. Its shape can be modified somewhat by deceleration
electrode arrays between the platform and ground.

More details of
the Low Energy Beam Transport (including the βNMR facility)optics
can be found here.